The present invention relates to methods and devices for sensing the presence and characteristics of coins as part of, for example, a coin-operated parking meter. Primary objectives of such devices, are to discriminate between valid coins and counterfeit ones or other coin-like objects, as well as between different denominations of valid coins.
A common coin sensing method employed by previous devices is the use of a sensor coil whose impedance is changed by the nearby presence of a metal object such as a coin. One type of discrimination circuit using such a sensor coil is a bridge circuit which includes standard impedance elements in addition to the coil. Passage of the coin near the coil then causes the balance point to change. Another type of detection circuit uses the coil as part of an oscillator circuit. The presence of a coin near the coil causes the frequency at which the oscillator resonates to shift. By measuring the frequency shift it is possible to detect the presence of a coin. Furthermore, the magnitude of the frequency shift will depend on such things as the size and material content (e.g., iron, copper, or silver etc.) of the coin. Therefore, standard frequency shift "signatures" for valid coins can be ascertained allowing the circuitry to discriminate between denominations of valid coins and between valid coins and other objects.
A problem with sensor coils of the type described above, however, is that the change in impedance (and frequency shift) is dependent upon both the total metal mass of the coin and the particular material out of which the coin is made. This means the same impedance change can be caused by either a large, low response material (e.g. copper) coin or a small, high response material (e.g. iron) coin.
Previous sensor coils may also require a large amount of power in order to properly discriminate between coins. This can be a particular problem in applications where the coin sensing device does not have access to an external power source.